The invention concerns a defibrillator comprising an implantable housing and an implantable electrode set which includes at least a right-ventricular electrode and a coronary sinus electrode, wherein arranged in the housing is a control device for the electrodes of the electrode set.
It is known that certain cardiac palpitations or arrhythmia phenomena including in particular ventricular and atrial fibrillation but possibly also accelerating tachycardia phenomena which have not yet passed into the state of fibrillation are to be electrotherapeutically treated with good prospects of success by applying short-duration electrical pulses (shocks) to the sensitive cardiac tissue. In order rapidly to achieve termination of such life-threatening arrhythmia effects with a high level of certainty, high energy levels are applied to the cardiac tissue (myocardium), which in many cases results in tissue damage and severe stresses such as pain for the patient. In relation to in particular implantable units, the provision of those high levels of energy requires particularly powerful batteries and capacitors. Those energy storage means are primarily crucial in regard to the structural size of implantable defibrillators. A reduction in the amount of energy required for the shocks permits smaller energy storage means to be used and thus smaller defibrillators. It is known that the energy required can be reduced by an advantageous design configuration in respect of the electrodes and an appropriate actuating mode. U.S. Pat. No. 5,224,476 describes a defibrillator of the general kind set forth, which aims to achieve a reduction in the amount of energy required. That implantable defibrillator has a control device which is arranged in a housing and an electrode set which includes an electrode arranged in the right ventricle, an electrode arranged on the coronary sinus, an electrode arranged in the vena cava, and a patch electrode which is arranged at the apex of the heart or subcutaneously. The control device is designed in such a way that those four electrodes are switched in two pairs and a re successively operated with altering polarities. A disadvantage with this known defibrillator is that it involves an irregular distribution of the electrical field. In addition implantation of the four electrodes of which at least one is a patch electrode which is particularly complicated in terms of implantation is time-consuming and stressful for the patient.
The object of the present invention is that of providing a defibrillator of the kind set forth in the opening part of this specification, which requires less energy for the defibrillation procedure and which is easier to implant.
The way in which that object is attained is set forth in the features of claim 1. Advantageous developments are recited in the appendant claims.
In accordance with the invention, in a defibrillator comprising an implantable housing and an implantable electrode set which includes at least a right-ventricular electrode and a coronary sinus electrode, wherein arranged in the housing is a control device for the electrodes of the electrode set, it is provided that the housing is conductive and is connected as an electrode. The invention is based on the notion that an electrode is formed by the housing of the defibrillator and there is therefore no need now for a patch electrode which is complicated in terms of implantation. The electrode configuration according to the invention provides that an electrical field which is produced upon the output of a shock is more uniformly distributed and is thus effective in a larger region with the same energy used for the field. The energy applied is thus utilised more efficiently. This greater efficiency makes it possible for only a lower level of energy to be sufficient for a shock which is adequate for defibrillation purposes. In practice energy levels of between 4 and 6 joules, preferably about 5 joules, have proven to be adequate to implement defibrillation with sufficient reliability. The defibrillator according to the invention also affords the advantage that it is easier to implant as an electrode is formed by the housing and only two electrodes which are separate from the housing and which are to be inserted directly in the heart are required; in addition there is no longer any need for a patch electrode which is particularly complicated and time-consuming to insert. This means that the implantation operation is easier for the surgeon and causes less stress for the patient. The expression conducting housing is used to mean not only that the housing is formed from an electrically conducting material, but also that the housing comprises a non-conducting material which is provided with a conducting layer.
A defibrillator is admittedly already known in which an electrode is formed by the housing (B. Ken Knight et al xe2x80x98Dual shock defibrillation with a new lead configuration involving an electrode in the left posterior coronary veinxe2x80x99, PACE, Vol 21, April 1998, page 806), but that defibrillator has a different electrode configuration which does not include any coronary sinus electrode but instead a distal electrode which is disposed substantially deeper in the ventricle. Upon shock output that different electrode configuration produces a completely different field configuration which involves a much lower level of efficiency than the field configuration which is afforded by the teaching in accordance with the invention; the consequence of this is that at about 12 joules, a good double the amount of energy is required for the defibrillation procedure, as in the case of the defibrillator according to the invention. In addition the housing electrode in the case of the known defibrillator serves a different purpose from the teaching according to the invention. More specifically, in the case of the known defibrillator, the housing electrode only serves to build up a field for pre-excitation, together with the distal electrode which is disposed deep in the ventricle. The configuration thereof differs from the configuration of the actual shock field which occurs between the other electrodes. The production of that pre-excitation field requires additional energy and is a cause of the high energy requirement of that previously known defibrillator.
Desirably provided as a further electrode is a vena cava electrode which is electrically conductingly connected to the electrode formed by the housing. Particularly uniform distribution of the electrical field is achieved by virtue of this additional electrode which is arranged in thexe2x80x94preferably superiorxe2x80x94vena cava and which, by virtue of the conducting connection to the housing, is at the same potential as the housing electrode. Large areas of the myocardium can be affected by the electrical field with the vena cava electrode, while in addition only low levels of leakage losses occur. The latter affords the advantage that unwanted stressing of the ambient tissue by the electrical field does not occur or occurs only to a slight degree.
Desirably, arranged in the housing is a shielding or screening cage which surrounds the control device. The screening cage acts as a Faraday cage and protects the control device from possible negative effects of the electrical fields which are produced upon shock output. The term cage is used to also denote those enclosures which are substantially closed except for individual openings. It will be appreciated that the screening cage comprises a material which is a good conductor. Advantageously, the screening cage is also such as to afford shielding from magnetic fields.
In accordance with a particularly advantageous embodiment the control device is connected in such a way that it operates the electrodes with a shock which has at least three phases and at least one change in polarity, with the electrode being actuated in all phases. The term change in polarity is used to mean that the electrode (or electrodes) which are connected as a cathode in one phase are connected as an anode in a following phase, and vice-versa. The fact that an electrode is actuated in all phases means that there are no completely separated field configurations in respect of the electrical field, because of the common electrode, as viewed over all phases. As the field patterns partially overlap, there is an implicit pre-excitation effect in the regions which are in the field configuration of a plurality of phases. This permits a further reduction in the amount of energy required to implement defibrillation.
Preferably the control device is switched in such a way that in a first phase a first electrode of the electrode set and a second electrode of the electrode set are actuated, in a subsequent phase the first electrode and a third electrode of the electrode set are actuated and in a further subsequent phase the first electrode and the second or third electrode are actuated with the reverse polarity. A three-phase mode of actuation of that nature, together with the electrode configuration according to the invention, has the advantage of affording a uniform distribution of the electrical field upon shock output, such as to avoid local peaks. It is particularly preferred if the right-ventricular electrode is the first electrode, the coronary sinus electrode is the second electrode and the housing is the third electrode. That affords in the first phase a concentration of the field and therewith the field energy in the ventricle region of the heart. That has the advantage that losses due to a field pattern outside the ventricle can be substantially avoided. A further advantage is that the ventricle is already depolarised thereby in the first phase. As the housing acts as an electrode which is extended in terms of surface area, the arrangement affords a uniform field distribution which is advantageous for acting on the entire ventricle. It has surprisingly been found that, in spite of the large field-filled space which is governed by the arrangement, remote from the heart, of the counterpart electrode in the form of the housing, only a lower level of energy is required than ifxe2x80x94as in the known defibrillatorxe2x80x94the more closely disposed vena cava electrode is the counterpart electrode.
Desirably the control device is switched in such a way that it actuates the electrodes with a shock which involves five phases. That can provide an even more uniform distribution of the electrical field over the myocardium. Preferably the five-phase shock, originating from the three-phase shock, is formed in such a way that phases one and two are the first two phases of the three-phase shock, phases three and four are a repetition of the first two, and the phase five corresponds to the third phase of the three-phase shock. It can also be provided that repetition of the first two phases includes a change in polarity. The five-phase shock has the advantage, in particular in conjunction with the electrode arrangement according to the invention, that the even more uniform distribution of the electrical field further minimises the losses and/or the stressing of the surrounding tissue by the electrical field.
Frequently, the control device is switched in such a way that the phases of a shock are of the same or at least an approximately identical time duration. The control device however can also be switched in such a way that the time durations of the various phases are different. Preferably then the duration of the first phase which is frequently also referred to as the phase zero is less than that of the following phase.
In an advantageous embodiment of the present invention the control device is so switched for the output of an atrial shock that it actuates the coronary sinus electrode and the vena cava electrode. In that way both atrial flutter and also atrial fibrillation can be specifically terminated by the output of an atrial shock, more specifically in such a way that the electrical field which is produced upon output of the shock is concentrated on the atria. The concentration effect both reduces the amount of energy required for the shock and also avoids unnecessarily stressing the rest of the cardiac tissue by the electrical field, or at least at any event alleviates it.
Preferably in that respect the control device is switched in such a way that it actuates the housing electrode together with the vena cava electrode. The fact that the vena cava electrode and the housing electrode are then at a common potential provides for uniform involvement of both atria with the electrical field which is produced upon output of the shock.